This invention is directed to an engine throttle control apparatus for a vehicle.
It is conventional practice to control the idling speed of a vehicle to an optimum value by adjusting the degree of opening of an auxiliary air control valve provided in an auxiliary air passage that bypasses the main throttle valve under idling conditions.
Some vehicles have a boost control valve function (that is, a negative pressure control function during deceleration) in addition to such an idling speed control function. The boost control valve function entails introducing air into the intake manifold to prevent excessive intake manifold vacuum during deceleration when the throttle valve is being closed. This is effective to limit the vacuum (or negative pressure) in the cylinder so as to reduce oil leakage into the cylinder and minimize other problems.
For such a boost control valve function, the degree of opening of the auxiliary air control valve is set as follows. A controlled variable BCV is stored in a table according to the engine speed. Control is made with reference to this table during deceleration. A comparison is made between a controlled variable ISC (used for idling speed control) and the controlled variable BCV (for negative pressure control). A greater one of these controlled variables is selected to control the air intake. Further details regarding such control techniques are set forth in U.S. Pat. No. 4,951,209 and Japanese Patent Kokai Nos. 1-121536 and 1-294933.
In recent years, torque demand control has been suggested to control a throttle valve in an electronic manner (U.S. application Ser. No. 08/804,454, filed Feb. 21, 1997, now U.S. Pat. No. 5,931,138, which disclosure is incorporated by reference into this application). Such torque demand control entails calculating a target engine torque based on accelerator position and other parameters (for example, accelerator position and engine speed), and calculating a basic fuel delivery requirement value in a manner so as to realize the target engine torque. A target intake air flow rate is calculated based on the basic fuel delivery requirement value and a target air-fuel ratio. A target throttle valve position is calculated based on the target intake air flow rate, and the throttle valve is controlled to the target throttle valve position.
It is possible to realize negative pressure control during deceleration with the use of an engine throttle control apparatus employing an electronic controlled throttle valve, by applying a lower limit to the target intake air flow rate calculated during torque demand control such that negative pressure control during deceleration sets a lower limit on the amount of intake air.
That is, negative pressure control can be realized by calculating a negative pressure control target intake air flow rate for negative pressure control during deceleration, after the target intake air flow rate is calculated for torque demand control, and by selecting a greater one of the negative pressure control target intake air flow rate and the target intake air flow rate based on torque demand, and calculating the target throttle position based on the selected rate.
However, under a condition where the target intake air flow rate corresponding to the torque demanded by the driver increases from a very small initial value, if a control unit is designed not to increase the torque in accordance with the driver demand until the lower limit (negative pressure control target intake air flow rate) is reached, the following problem can occur. At the time t1 in FIG. 1, the engine torque does not increase (and the driver feels no acceleration) in spite of the driver demand for increased torque. At the time t2, the feeling of deceleration is rapidly eliminated in spite of no change in the driver demand.
In other words, at times tl and t2, the vehicle does not act in accordance with the driver's demand and expectations. The time between t1 and t2 can be up to several seconds. Thus, such a control technique results in non-optimum driving performance.